We have developed a method for purification of leucocyte pyrogen from rabbit peritoneal exudate cells. We have also been able to obtain endogenously labelled pyrogen by incubating cells in the presence of labelled amino acids and purifying pyrogen from the crude cell supernates. We now wish: (1) To obtain an antibody to leucocyte pyrogen which will be used (a) in conjunction with labelled antigen, to develop a radioimmuno assay which can measure pyrogen content in body fluids. If this is successfully achieved, the method may prove useful in clinical fevers, because human pyrogen causes fever in rabbits (b) with a fluorescein or peroxidase label to study when and where pyrogen is synthesized in the neutrophil polymorph. (2) To investigate whether pyrogen is bound by hypothalamic neurons. Endogenously labelled pyrogen is not sufficiently active to be a useful reagent; we will have to use pure pyrogen labelled with I125. (3) To determine the primary structure of leucocyte pyrogen. If the known molecular weight can be completely accounted for by amino acids. This will establish that leucocyte pyrogen is a true hormone produced by inflammatory cells, and is not in any way derived from bacterial endotoxin. We would also like to split the pyrogen molecule using reagents such as cyanogen bromide in the hope that a portion of the molecule will prove to carry all the biological activity. If this is the case, and the fragment is small enough, it would be possible to synthesize it, and to abandon the present tedious methods of producing and purifying pyrogen. (4) To study the mechanisms by which pyrogen is released from granulocytes. There is a great deal of confusing evidence about the effect of various ions and metabolic inhibitors on the process; we have preliminary data suggesting that variations in cyclic AMP levels explain these effects.